(1) Field of the Invention
The present invention relates to a heat-resistant rubber composition. More particularly, the present invention relates to a rubber composition comprising a modified or unmodified ethylene/.alpha.-olefin copolymer rubber and a specific silane-treated silica type filler or a combination of a silane-treated or untreated silica type filler and a specific polyorganosiloxane, which gives a cured rubber excellent in mechanical properties, resistance to heat aging and electric properties.
(2) Description of the Prior Art
An ethylene/.alpha.-olefin copolymer rubber is widely used as a rubber having excellent mechanical properties and electrically insulating properties for car parts, electric parts, ordinary industrial parts, and the like. Recently, a high resistance against heat aging is often required for car parts, electric parts and the like.
Accordingly, as is shown in Complete Book of Synthetic Rubber Processing Techniques, "Ethylene-Propylene Rubber", addition of antioxidants to rubbers of this type has been tried, but no satisfactory improvement effect can be attained.
A composition comprising an ethylene/.alpha.-olefin copolymer and a silica type filler has been used in various fields where a rubber is ordinarily used. It is known that if a silica type filler is surface-treated with trimethylchlorosilane or the like, a hydrophobic property can be imparted, and this silane-treated silica type filler is incorporated in an ethylene/.alpha.-olefin copolymer.
A composition of this type is excellent in the initial physical properties, but the aging resistance is still insufficient.
It has already been known that reinforcing silica and an organopolysiloxane are incorporated in an ethylene/.alpha.-olefin copolymer such as EPR. For example, Japanese Patent Application Laid-Open Specification No. 116739/81 and Japanese Patent Application Laid Open Specification No. 120744/81 teach that organochlorosilane-treated silica and an organopolysiloxane such as dimethylpolysiloxane are incorporated in EPR, and Japanese Patent Application Laid-Open Specification No. 195757/82 teaches that finely divided silica and an organopolysiloxane are incorporated in EPR.